Synapse formation and elimination are fundamental to the assembly of neural networks during development and give rise to nearly every cognitive process. Neurexins are presynaptic cell-adhesion molecules whose polymorphisms have been implicated in autism and schizophrenia. Reduced systems have strongly implicated neurexin as a potent synaptogenesis molecule, but no phenotype in synaptogenesis was observed in ?-neurexin KO studies; this result suggests that there may be compensation for ?-neurexins. This research proposal aims to 1) develop and analyze novel conditional knockout mice in which both the ?- and -forms of all 3 neurexin genes can be deleted by Cre recombinase, and 2) use this novel mouse model to investigate fundamental requirements for neurexins in synapse formation and function. Specifically, synapse formation will first be determined in vitro in culturd neurons by morphological and electrophysiological approaches, and subsequently in vivo at the olivo-cerebellar synapse, which provides a unique and elegant preparation to examine synapse formation, development, and maintenance. The proposed studies will test the following hypotheses: whether the complete knockout of neurexins (Nrxn1/2/3 ?/) will 1) decrease the frequency, but not amplitude, of spontaneous miniature synaptic potentials, and lead to the loss of active synapses; 2) decrease synapse numbers as measured from morphological analysis in culture systems; and 3) lead to the failure of synapse elimination or maturation in the developing climbing fiber-Purkinje cell synapse. These experiments provide the first direct test of the hypothesis that neuron-specific adhesion molecule, neurexin, plays the crucial role in synapse development and synaptic functions. The conditional mouse model is so-far the most complete and approachable system for neurexin, mainly due to neurexin's critical biological importance, where knocking out only the ?-form of neurexin is postnatally lethal, thus limits research approaches. It is projected that the results will expand extensively to our understanding of neurexin actions in brain and will provide valuable insights to this fundamental hypothesis. Furthermore, the proposed work hopefully would produce insights into the etiology of neurocognitive diseases/disorders based on neurexins' polymorphism that is strongly implicated in autism spectrum disorders, schizophrenia, and other cognitive diseases. Finally, the proposed project will provide outstanding training opportunities to develop my professional qualifications in pursuing an independent academic career that requires a multidisciplinary approach.